Sheet fed machines, such as corrugated sheet printing machines, consist of multiple operating sections. The operating sections will typically include a feed section and one or more processing sections, such as a print section(s), a die cutter section, a slotting section, and the like. The actual specific number and types of sections will depend on the task that is being performed. Each operating section includes a pair of cylinders where each cylinder in the pair rotates in opposing directions. In the processing sections, at least one cylinder (the “processing cylinder”) includes a tool.
The term “registration” is commonly used in the relative technical fields to mean the proper alignment of various plates, cylinders, or the like to assure clear and accurate reproduction and manufacturing of a sheet or web. For a sheet fed machine, registration can be defined as the control and alignment of the machine's processing cylinders to provide consistent and accurate printing, cutting, slotting, and/or other process on the processed sheet. A common gear train links the cylinders in the operating sections. The processing cylinders have register or compensator motors that rotate the cylinders without rotating the gear train in order to register the cylinders.
In reference to corrugated sheet printing, the machines are largely responsible for converting a corrugated sheet into a more aesthetically pleasing and marketable product, such as boxes with printed images. Corrugated sheets, in general, are paper or plastic sheets that consist of multiple layers. Most commonly, there are two flat, outer layers sandwiched around at least one inner layer. The inner layer(s) is corrugated, meaning it is shaped into alternating parallel grooves and ridges. The wave-like pattern of the middle layer(s) (i.e., the alternating ridge/groove shape) provides adhesion or connection points at the apex of the ridges and grooves to be joined to adjacent flat panel layers. Adhering or joining a lone middle layer to the two outer layers would form a corrugated sheet with a single inner layer. Intermediate flat layers could be used where there is a plurality of inner corrugated layers.
A feed mechanism advances a new sheet into the machine. Often, a stack of sheets is placed on the feeding unit or mechanism wherein the bottom sheet is propelled into the printing machine. Some feed units are capable of feeding two sheets per machine revolution, but it is more common that a single sheet is inserted. Each sheet is advanced so that it engages the cylinders of the feed section. The specific angular position of the cylinders in the feed section at the time of contact with the individual sheets is unimportant. However, the sheets should be delivered to the processing cylinders so that each sheet reaches the processing cylinders when they are at a specific angular position. The position is critical as it determines where the tool on the cylinder engages the sheet as the sheet passes between the cylinder pair. When properly registered, the sheet reaches the processing cylinder pairs so that any printing, cutting, or other processing on the sheet is properly located on the sheet.
The registration, however, often requires adjustment for a variety of reasons. For example, after a finite production run of sheets, the machine must be opened and new print plates and die boards attached to the processing cylinders in accordance with the specific specifications of the next production run. This requires an operator to rotate the cylinders within the machine without rotating the gear train. The cylinder must be returned exactly to the previous position to “re-register” the machine. However, the print plates are not uniformly sized and may be mounted improperly.
A more significant problem occurs when the operator rotates the gear train to move a specific cylinder. The gear train of a section is disconnected from the adjacent sections when the machine is open. When the operator moves the gear train for one section, it becomes tedious and/or difficult to accurately mesh the gear train back together to the exact location as it was before. This problem is well known in the art of sheet fed machines because moving the gear train typically results in the machine being “out of time.” The initial registration is almost random when the machine is out of time and will likely require many more adjustments to reach the proper registration.
As an illustration of registration, consider the example of a sheet measuring 20 inches by 20 inches that might need to have a simple image printed in the center of the sheet and then to be cut into the shape of a circle wherein the printed image is in the center of the resulting circle. Sheets would first be stacked onto or otherwise provided to the feed mechanism. A print plate with the desired image and color would be mounted onto one of the print sections (plates in additional print section would be used to blend colors, add secondary images, or the like). A die board is mounted onto the die cylinder, and the die board would have a knife for cutting or scoring a circular shape into the sheet. The machine is started, and the sheets are individually fed into the machine. There is almost always some level of registration error. The machine is properly registered when the image is placed in the appropriate position so that when the sheet is cut the image is in the middle of the circle.
In known registration systems, in order to achieve a properly registered product, the print cylinder and die cylinder would be adjusted by means of an electric motor (commonly referred to as a register motor or compensator motor). The function of this motor is to adjust the rotational position of the cylinder at the time when the cylinder pair engages a sheet fed by the feed mechanism. A register dial rotates with the cylinder to give a visual indication of how much rotational movement of the cylinder has occurred (encoders can replace the dial to give an electronic indication). Each section is equipped with a “timing dial” or digital readout of each section's relative gear train position. If the gear train of a section is moved independently, the timing dial and register dials move. By changing the relative position of a print cylinder, die cylinder, or slotting cylinder relative to gear train and/or by changing the relative position of a section's gear train relative to other gear train sections, it is possible to register a particular product to the desired specifications.
Currently, registration is accomplished by processing sheets in the machine and then making adjustments as necessary based on the error found in the resulting product. Using the above example, if the first sheet produced by the machine includes an image that is 6″ away from the center of the circle, the operator would adjust the register on the print cylinder to advance or retard the print on the sheet. The same could be done to the die cylinder, if necessary. For example, the die cylinder would need to be adjusted in the event that the circle was truncated because the knives were not properly falling within the area defined by the sheet.
The known registration techniques obviously require the operator to inspect the end product and then to walk to the unregistered processing section in order to adjust the dials. The adjustment is usually an estimate meaning multiple runs are required to perfect the registration. Multiple processing sections may require registration. This is a time consuming and wasteful process.
A machine is thought to be “in time” when it is possible to set the register dials to zero (which indicates the position of the processing cylinders relative to the corresponding section's gear train), to have all the timing dials at zero (which indicates the section's gear train position relative to other sections' gear train position), and have the first sheet fed into the machine such that the sections print, die cut, and/or slot the sheet in a manner that is reasonably close to what the manufacture intended. Many variables, as known to those in the art, make it nearly impossible to have a first sheet exactly right even with the machine is in time.
For instance, the timing and register dials inherently provide poor resolution, are subject to human error, or are generally not accurate. If the machine is “out of time” (meaning the timing dials are not properly calibrated), the process of finding the correct registration becomes almost random since there is no reasonable way to know the rotational position of each cylinder relative to the sheet feeding mechanism. It may be necessary to open the machine and adjust the gear train until the machine is ‘in time’.
Web-based machines, for various reasons, allow for automatic registration and pre-registration of machines, but this has not been accomplished in sheet fed machines. Registration systems for sheet fed machines rely on encoders, which are geared electrical devices that generate signals that can be decoded to determine a relative angular position of a cylinder. These systems require periodic human “referencing” since they tend to lose signal pulses or generate false pulses. Very importantly, encoder-based systems do not account for a machines gear train being out of time. The gear train can be moved without the encoder knowing that the position of the cylinder relative to other sections has been changed by the amount of rotational movement that occurred in the gear train. Even fractions of an inch render the machine out of time, and the position reported by the encoder is completely inaccurate.
Known systems cannot track, record, or use the elapsed time between various sensor signals generated from the sections of the machine. The ability to rely on elapsed time between the various sensor signals in order to determine the relative positions of each cylinder would be advantageous. Manufacturing inconsistencies, human error in mounting the flexible plates or boards on the cylinders, and other factors that are specific to sheet fed machines create unique challenges for registering sheet fed machines.
Therefore, there exists a need and a challenge to create a registration system for sheet fed machines, such as corrugated sheet printing machines that provides a very simple and economical way to display the current relative positions of each cylinder. Ideally, such a system would allow a sheet fed machine to be registered even without the machine being “in time” and would be operable to display a fixed point on each cylinder relative to the feed mechanism. As such, calibration of the time/registration dials is effectively eliminated. The ability to perform manual registration adjustments would be provided. The registration system would provide the relative position of the cylinders relative to the timing of a sheet feed mechanism using elapsed time. The registration system of the present invention solves one or more of these or other needs.